Isobutylene-based polymers, such as isobutylene-isoprene and isobutylene-paramethylstyrene copolymers, as well as halogenated variants thereof exhibit considerably lower air permeabilities than other elastomers, and this has led to their being the material of choice for the inner tubes and innerliners that act to retain the air pressure in nearly all modern pneumatic tires. However, there is a continuing need to improve the air retention characteristics of such components even further, in order to improve their performance in terms of energy efficiency and safety. One route to such improvements has been the synthesis of polymer-clay nanocomposites, wherein nanometer-scale clay sheets are dispersed within the polymer to lower their air permeability even further.
Nanocomposites are polymer systems containing inorganic particles with at least one dimension in the nanometer range, e.g. inorganic substances from the general class of “phyllosilicates”. Ideally, intercalation should take place in the nanocomposite, wherein the polymer inserts into the space or gallery between the clay surfaces. Ultimately, it is desirable to have exfoliation, wherein the polymer is fully dispersed with the individual nanometer-size clay platelets. Due to the general enhancement in air barrier qualities of various polymer blends when clays are present, there is a desire to have a nanocomposite with low air permeability, e.g. for use in the manufacture of tires.
The preparation of nanocomposites uses a number of methods to generate exfoliated clays. One of the most common methods relies upon the use of organically modified montmorillonite clays. Organoclays are typically produced through solution based ion-exchange reactions that replace sodium ions on the surface of sodium montmorillonite with organic molecules such as alkyl or aryl ammonium compounds and typically known in the industry as swelling or exfoliating agents. Among the deficiencies of this method can be the limited thermal stability of the ammonium compounds, the lack of chemical bonding with the matrix, often leading to poor mechanical properties and increased hysteresis, and the negative impact the released amines and degradation products have on the transport properties. WO 2004/058874 discloses a process of preparing nanocomposites from organically-modified clays, butyl rubber and a polymeric exfoliant.
Another method used in the art to improve the organoclay performance is to combine functionalized polymers with the clay. This approach has been limited to materials that are soluble in water or to materials that can be incorporated into the polymerization reaction. This approach has been used to prepare nylon nanocomposites, using for example, oligomeric and monomeric caprolactam as the modifier. Polyolefin nanocomposites, such as polypropylene nanocomposites, have utilized maleic anhydride grafted polypropylenes to achieve some success in the formation of nanocomposites.
Elastomeric nanocomposite innerliners and innertubes have also been formed using a complexing agent and a rubber, where the agent is a reactive rubber having positively charged groups and a layered silicate uniformly dispersed therein. However, this approach to improving air barriers has limited usefulness due to the need for pre-formed positively charged reactive rubber components.
Nanocomposites have also been formed using non-ionic, brominated copolymers of isobutylene and para-methylstyrene, and blends of these copolymers with other polymers. However, it has been found that the efficiency of clay exfoliation, as determined by the relative permeability reduction, is not as high as that achieved in routes involving ionic interaction. Nanocomposites made of clay and amino-functionalized halogenated elastomers are disclosed in WO 02/100935. Nanocomposites comprising an interpolymer and clay treated with an exfoliating additive are disclosed in WO 02/100936.
WO 2008/045012 discloses a process to produce a nanocomposite comprising the steps of mixing an aqueous slurry of clay with a solution of polymer in an organic solvent to form an emulsion comprising a polymer-clay nanocomposite, and recovering the nanocomposite from the emulsion. The polymer may be pre-functionalized e.g. with an amine group in order to increase interaction with the clay.
As described above, nanocomposites are made in the art by mixing of elastomers and organoclays either at the melt state or in solution; and, due to the hydrophobic nature of the polymer, the organoclays (and/or the polymers) are typically modified to provide better interaction between the clays and the polymers. This process is expensive and most modified clays are not exfoliated in polymers or in organic solvent.
Thus, there is still a need in the art for a process of preparing a polymer/clay nanocomposite with improved exfoliation of the clay and increased interaction between the clay and the polymer. There is also need for a less costly process to produce polymer/clay nanocomposites using inorganic clay without organic modification or without using polymer that has been pre-functionalized. Additionally, if the polymer is halogenated rubber, ideally, a process for preparing nanocomposites of clay and halogenated rubber should be capable of being integrated into the halogenated rubber production process. Finally, there is still a need in the art for polymer/clay nanocomposites having even better air barrier properties (i.e., lower oxygen transmission rates) than existing nanocomposites while maintaining good processability, and that can be used in applications such as tire innerliners where toughness and low air permeability are required.